The present invention relates to rare earth oxybromide phosphors activated with bismuth for conversion of X-rays to visible light and to X-ray image converter devices utilizing such phosphor materials. The present invention further relates to these phosphor materials having reduced afterglow or phosphorescence from incorporation of activator amounts of ytterbium ion in the lattice of the host phosphor.
Rare earth oxyhalide phosphors activated with terbium or thulium ion are more efficient luminescent materials for converting X-radiation to visible light than conventional phosphors now in use. For example, a LaOBr:Tb has about a 50% greater absorption of incident X-rays and about three to four times greater blue emission than conventionally used CaWO.sub.4 phosphor. The various X-ray image converter devices in which said luminescent material is used are described in U.S. Pat. No. 3,617,743 while U.S. Pat. Nos. 3,591,516 and 3,607,770 disclose various processes which can be used to prepare the luminescent material. On the other hand, current radiographic screens employ photographic film which tends to fog when this phosphor is used by reason of an afterglow or long persistent phosphorescence of appreciable intensity. With the use of such long afterglow phosphors, any movement of the film relative to the phosphor screen immediately after exposure will result in blurred images. Addition of ytterbium ion to this phosphor material virtually eliminates the problem of blurred images as described in the aforementioned U.S. Pat. Re 28,592. A different blue emission phosphor which is not subject to the afterglow problem results from activating the same phosphor host material with thulium as disclosed in the further above mentioned U.S. Pat. No. 3,795,814 patent. Said improved phosphor material has significantly reduced afterglow without an appreciable loss in phosphor efficiency.
While the above mentioned rare earth oxyhalide phosphors provide improved performance compared with the conventional CaWO.sub.4 phosphor, new phosphors having different characteristics are constantly being sought. It is also not possible to predict whether new phosphors will produce light efficiently since the light output depends upon complex optical and energy interactions. The ability of the host crystal to absorb the X-radiation and convert it into light which can be transmitted back out through the host material is thereby not subject to prediction as a general rule.